1. Field of the Invention
The present invention relates to a gas sensor, such as an oxygen sensor, HC sensor, or NOx sensor, for detecting a component in gas to be measured.
2. Description of Related Art
Conventionally, a gas sensor is known to assume a structure in which a metallic casing accommodates a bar-like or cylindrical sensing element which has a sensing portion formed at its tip end and is adapted to detect a component in gas. The metallic casing often includes a combination of a plurality of cylindrical members, such as a metallic shell, a protector, an inner cylindrical member, and an outer cylindrical member. The metallic shell has a screw portion formed on its outer surface which is used for attachment. The protector is connected to the metallic shell in such a manner as to cover the sensing portion of a sensing element which projects from one end of the metallic shell. The inner cylindrical member is connected to the other end of the metallic shell and adapted to cover the sensing element which extends rearward from the metallic shell; i.e., opposite the protector with respect to the metallic shell. The outer cylindrical member is connected to a rear end portion of the inner cylindrical member and allows a lead wire from the sensing element to extend rearward from a rear open end thereof.
Various methods are employed for establishing a bond between the plurality of cylindrical members. For example, when an airtight bond is required, an end portion of one member is loosely fitted or press-fitted into an end portion of the other member so as to form an overlap zone. Subsequently, the overlap zone is circumferentially welded, thereby forming a circumferential weld zone.
The above conventional bonding methods involve the following problems. In the case of, for example, a loose fit between the inner and outer members, if an excessively large gap is formed between the overlapped inner and outer members, the weld zone is apt to include an incompletely welded portion, failing to attain airtightness. In the case of a press fit, if the outside diameter of the inner member is excessively large, the practice of press fitting causes the outer member to be forcibly expanded, potentially resulting in flaring of the press-fitted open end portion of the outer member. As a result, a relatively large gap is formed between the inner member and the flared end portion of the outer member, potentially failing to form a complete weld zone.
Accordingly, when the inner member and the outer member are to be welded regardless of whether they are loosely fitted or press-fitted, their dimensions must be controlled in a considerably strict manner so as to avoid formation of an excessive gap between the fitted inner and outer members. However, employment of a relatively small dimensional tolerance accompanies labor-intensive, costly process control, causing an increase in sensor cost.
The present invention provides a gas sensor comprising a bar-like or cylindrical sensing element and a cylindrical casing. The sensing element has a sensing portion formed at a tip end portion thereof and is adapted to detect a component in gas to be measured (gas under measurement). The cylindrical casing covers the sensing element while the gas under measurement is permitted to flow therethrough to the sensing portion. The casing includes at least two axially adjacent cylindrical members. An end portion of one cylindrical member (an inner member) is disposed within a corresponding end portion of the other cylindrical member (an outer member) to thereby form an overlap zone. A diameter-reduced portion is circumferentially formed on the outer member in such a manner as to be located at an axially intermediate position of the overlap zone. A weld zone is circumferentially formed at the diameter-reduced portion so as to establish an airtight bond between the outer member and the inner member.
The present invention further provides a method for manufacturing a gas sensor comprising a bar-like or cylindrical sensing element and a cylindrical casing. The sensing element has a sensing portion formed at a tip end portion thereof and is adapted to detect a component in gas to be measured (gas under measurement). The cylindrical casing covers the sensing element while the gas under measurement is permitted to flow therethrough to the sensing portion. The casing includes at least two axially adjacent cylindrical members. The method comprises the steps of: disposing an end portion of one cylindrical member (an inner member) within a corresponding end portion of the other cylindrical member (an outer member) to thereby form an overlap zone; circumferentially forming a diameter-reduced portion on the outer member in such a manner as to be located at an axially intermediate position of the overlap zone; and circumferentially forming a weld zone at the diameter-reduced portion so as to establish an airtight bond between the outer member and the inner member.
According to the above-described structure of the gas sensor and the method for manufacturing the gas sensor, the casing that covers the sensing element includes the following structural features. The two cylindrical members are disposed in such a manner as to overlap each other in the axial direction of the gas sensor. The diameter-reduced portion is circumferentially formed on the outer member and located in the overlap zone. Through formation of the diameter-reduced portion, a gap formed between the outer member and the inner member as measured at the position of the weld zone can be reduced to thereby improve adhesion between the members, so that weld defects rarely arise. Accordingly, attainment of a good welded state does not require the practice of dimensional control of the inner and outer members; particularly, strict dimensional control of the difference between the bore diameter of the outer member and the outside diameter of the inner member. Hence, production efficiency or yield of gas sensors is improved.
The diameter-reduced portion may be formed into a band-like shape having a predetermined width along the circumferential direction of the outer member. The weld zone may assume an annular form located at an widthwise intermediate position of the diameter-reduced portion and having a width narrower than that of the diameter-reduced portion. As a result, the weld zone is continuously formed within the band-like diameter-reduced portion, in which through a reduction in the gap formed between the inner and outer members, adhesion between the members is improved. Hence, the possibility of a weld defect arising is further suppressed.
The weld zone is preferably formed by laser welding because of the reduced possibility of defects. The welding method is not, however, limited to laser welding, as resistance welding, such as seam welding, may be employed.
The diameter-reduced portion may assume the form of a caulked portion. The caulked portion is formed by circumferentially caulking the outer member toward the inner member in the overlap zone of the outer member and the inner member. Through employment of caulking, adhesion between the outer member and the inner member at the diameter-reduced portion can be further improved. Thus, the weld zone can be further effectively prevented from suffering a defect.
The inner member may have a concave portion which is located in the overlap zone and circumferentially formed at a position corresponding to the diameter-reduced portion of the outer member. For example, when the diameter-reduced portion of the outer member is to assume the form of the caulked portion, caulking may be performed to such an extent that the concave portion is formed on the inner member at the corresponding position. Through formation of the concave portion, adhesion between the outer member and the inner member is further improved, thereby reducing probability of a defect arising in the weld zone.
In the above-described gas sensor, the inner member may assume the form of a metallic shell which covers the sensing element while the sensing portion projects through one end portion thereof. The outer member may assume the form of a protector connected to an open end portion of the metallic shell through which the sensing portion projects, and is adapted to cover the sensing portion while gas under measurement is permitted to flow therethrough to the sensing portion. In the case of an oxygen sensor, for example, when sensor temperature decreases, water droplets may adhere to the outer surface of the protector due to condensation. In this case, if the weld zone bonding the metallic shell and the protector has a defect, the water droplets may enter through the defect and wet the sensing portion, or may cause dirt, such as rust, to adhere to the sensing portion. However, through application of the present invention to formation of the weld zone, the weld zone is effectively prevented from suffering a defect, so that such a problem is less likely to arise.
The inner member may assume the form of an inner cylindrical member, one end of which is connected to an open end portion of the metallic shell opposite the open end portion through which the sensing portion projects, the inner cylindrical member being adapted to cover the sensing element extending rearward from the metallic shell. The outer member may assume the form of an outer cylindrical member connected to the exterior of a rear end portion of the inner cylindrical member while a lead wire from the sensing element extends out through a rear open end portion thereof. In the case of an oxygen sensor for use in an automobile, for example, the outer cylindrical member or the inner cylindrical member is exposed to the exterior of the automobile. For example, when the oxygen sensor is mounted near a suspension and tires, water is apt to splash over the exposed member. As a result, if the weld zone of the inner and outer members has a defect, water may enter through the defect into the interior of the inner cylindrical member and cause malfunction of the sensor. However, through application of the present invention to formation of the weld zone, the weld zone is effectively prevented from suffering a defect, so that such a problem is less likely to arise.
An object of the present invention is to provide a gas sensor which includes a casing having at least two welded cylindrical members and which is less susceptible to weld defects without requiring the practice of considerably strict dimensional control of the cylindrical members, as well as to provide a method for manufacturing the gas sensor.
Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiments and appended drawings wherein like reference numbers refer to the same component, element, or feature.